Effect Of Gamma-Ray Irradiation On Charge Transport Behavior Of Polyethylene Terephthalate Film

With the rapid development of nuclear power technology,the safety of electrical equipment in nuclear power plants has received extensive attention.Polymer insulation materials have been widely used in radiation environments such as nuclear power plants.Therefore,it is of great significance to understand the ageing mechanism in irradiation environment to ensure the safety of nuclear power equipment.In this thesis,polyethylene terephthalate film(PET)was employed as test sample to investigate the effect of gamma-ray irradiation on charge transport behavior of PET film.In addition,the physical and chemical structure,mechanical properties and simulation had been employed to assist the analysis of the correlation between material structure and charge transport behavior,which provides theoretical and experimental basis for the safe use of PET in nuclear power plants.The main works are as follows:1.The PET film was irradiated by a ⁶⁰Co gamma-ray source,and the total doses were 0,100,500 and 1000 k Gy,respectively.Based on the theory of polymer chemistry,the physical and chemical structure of the PET film under different irradiation doses was measured.It was found that no significant change was observed in the Fourier Transform Infrared Spectrum of the PET film after irradiation.The oxidation reaction of the PET film was observed by X-ray Photoelectron Spectroscopy and Ultraviolet-Visible Spectroscopy,and oxidation products such as carboxyl and hydroxyl groups were deduced.The value of optical band gap(E_g)was decreased after irradiation.There was no cross-linking reaction occurred in PET film under irradiation.As the irradiation dose increased,the melting point,the initial melting temperature,the crystallinity and the lamellar thickness of the PET film overall increased,while the melting range and crystallization temperatures decreased.The size and regularity of crystallite of the PET film increased with the irradiation.2.The surface potential decay method was used to study the effect of?-irradiation on the charge transport behavior of PET films.It was found that with the increase of the total dose,the deep trap center for electron or hole of PET film was basically unchanged and the shallow trap center became shallower,while the density of deep and shallow traps center decreased.The positive and negative charge mobility increased with increasing irradiation dose,and the charge transport behavior comformed to the hopping model.The DC conductivity of PET film samples has obvious temperature dependence.At 80°C,the DC conductivity increased with the increase of the total irradiation dose,meanwhile at 35 and 150°C,the conductivity did not change significantly.The DC breakdown strength of PET decreased under the irradiation.With the increase of the total dose,the tensile strength increased first and then decreased,which was consistent with the change of crystallinity,while the elongation at break decreased.3.The unipolar charge transport model was used to simulate the surface potential decay process.The results showed that the experimental results of the surface potential decay were in good agreement with the simulation,which proposed unipolar charge transport model was capable of predicting the decay characteristics of PET samples before and after irradiation.Moreover,through theoretical calculations,the effects of bulk trap density,bulk trap center and surface trap center,depending on the irradiation dose,on the surface potential decay curve were analyzed by simulation.The results showed that with the gamma-ray irradiation,the bulk trap density was reduced,the bulk trap center became shallower,the surface trap center became shallower and the temperature rose,which changed the charge injection,the trapping/de-trapping process and transport process,resulting in the increase of surface potential decay.